Middle atmosphere

Noctilucent clouds in the Southern Hemisphere are dimmer, less frequent and higher than those in the Northern Hemisphere
Noctilucent clouds in the Southern Hemisphere are dimmer, less frequent and higher than those in the Northern Hemisphere (Photo: John French)

The middle atmosphere is divided into regions based on its temperature structure. Upward from the ground these are:

  • troposphere
  • stratosphere
  • mesosphere and
  • thermosphere

See the middle atmosphere diagram

The troposphere and tropopause

In the lowest 15 km of the atmosphere, temperature decreases with altitude at a nearly constant rate (the so-called lapse rate, which is approximately −6°C per kilometre). In this region, called the troposphere, most of what is termed ‘weather’ occurs. The troposphere is characterised by convective motion. Water vapour, aerosols and the ‘greenhouse gases’ dominate the climate processes.

Tropospheric temperatures maximise near the equator and decrease towards each pole. At the tropopause, the upper boundary of the troposphere, the lapse rate changes sign, that is, the temperature goes from decreasing with height, to increasing with height. This transition is least pronounced at mid-latitudes. The tropopause is highest in the tropics (~16 km) and is lowest in the polar regions (~8 km).

The stratosphere and stratopause

From the tropopause to about 50 km up, temperature increases with altitude. In this region, termed the stratosphere, only weak vertical motions of air occur and radiative processes provide the dominant energy flow. The increase in temperature with altitude is due to heating by ozone absorption of solar ultraviolet (UV) radiation.

Stratospheric temperatures are highest over the summer pole and decrease steadily to a minimum over the winter pole. The region at the top of the stratosphere, where temperature starts to decrease with altitude, is called the stratopause.

The Antarctic stratosphere is cold enough for clouds to form in spring. Stratospheric clouds provide surfaces on which chlorine from chlorofluorocarbons (CFCs) catalytically destroys ozone in the presence of sunlight.

Like all atmospheric regions, the stratosphere contains aerosols. In general, the highest concentration of aerosols is confined to below an altitude of 30 km. Aerosols play a major role in atmospheric chemistry, contributing to the nucleation of water droplets forming clouds, and the cooling of the atmosphere through re-radiation.

Aerosols in the stratosphere play a major role in the cooling of the lower atmosphere and assist in the destruction of ozone. A portion of stratospheric aerosols are natural in origin, coming from sources such as meteor ablation and volcanic events. The remainder are of human origin, coming from sources such as CFC emissions and high altitude aircraft exhausts.

The mesosphere

From the stratopause up to about 85 km (95 km during the winter), is the region known as the mesosphere. Temperature decreases with altitude as ozone heating diminishes. In this region, convective and wave motions and radiative processes are both important in transporting energy.

Temperatures are coldest over the summer pole and increase steadily to a maximum over the winter pole. The region at the top of the mesosphere is termed the mesopause.

The mesopause

The mesopause is the coldest region of the Earth's atmosphere (ranging between −150°C in winter and −90°C in summer). It is sufficiently cold for noctilucent (‘night shining’) clouds to form in summer, at altitudes around 83 km. The altitude of the mesopause varies with season and latitude. In the polar regions, the altitude of the mesopause ranges between 90 km in summer and 110 km in winter. Atmospheric physicists suspect the mesopause is very sensitive to climate change. Climate change variations are warming the troposphere and this is thought to be causing further cooling in the mesosphere.

The thermosphere

The region above the mesopause is called the thermosphere. Thermospheric temperatures are strongly influenced by solar activity.

Climate change

Increased concentrations of greenhouse gases such as carbon dioxide result in higher temperatures in the lowest reaches of the atmosphere, but cooling of greater magnitude in the upper atmosphere. The swap-over from heating to cooling depends principally on atmospheric density and occurs at about the height of the tropopause (10–20 km).

Some measurements of the middle atmosphere have reported cooling rates 5 to 10 times greater than expected from our present understanding.

The polar upper atmospheres may be regions where ‘greenhouse warming’ can be most definitively demonstrated. Whether this is so depends on the magnitude of the cooling compared to the natural variability. This is a subject of Australian Antarctic Division study.